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What is trend in a time series and how is it different from seasonality?
Trend is the long-term direction of a series, such as steady growth or decline, while seasonality is a regular repeating cycle within the year or week. A business can have rising sales (trend) that still peak each December (seasonality). Good forecasts separate the two so one is not mistaken for the other.
Methods like decomposition split a series into trend, seasonal and remainder parts so you can see each one. The remainder is what is left over, similar to the residual in a model.
Be cautious about extrapolating trends far ahead. Growth rarely continues in a straight line, and a trend fitted to a short period may be a temporary swing. Consider damped trends that flatten over time, and compare with an assumption of no trend.
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